1. Field of Invention
The present invention relates to image processing, and more particularly a system and method for generating image processor screens that are useful in copy machines and printers.
2. Description of Related Art
When color images are output to paper using primary color dot patterns, a slight offset in the angle of the primary color dot patterns can result in interference stripes known as "moires". For this reason, in the field of screen printing, screen angles are assigned beforehand to the screens for each of the primary colors. For example, a screen angle of 0 degrees could be used for yellow, 15 degrees for cyan, 45 degrees for black, and 75 degrees for magenta.
Japanese Examined Patent Publication Number 52-49361 and Japanese Examined Patent Publication Number 54-18302 disclose the implementation of this technology in digital circuitry. In the technology disclosed in these publications, a half-tone cell comprises twenty to several hundred image elements, and a repeating tile roughly in the shape of a square comprises a plurality of these half-tone cells. Halftones are reproduced based on the number of on and off image elements within the halftone cells.
Each image element in a halftone cell is assigned a prescribed threshold value, and each image element is turned on or off based on whether the threshold value is higher or lower than the image data. An example of this process is shown in FIG. 9, which shows a case where the level of input image data is 182.
The halftone cell here corresponds to a cell from the screen to be used in screen printing, so a screen angle is assigned to each primary color. However, in small-scale digital circuits, it is useful to use rational numbers for the tangents of the screen angles. Thus, if a screen angle is 15 degrees or 75 degrees (ideal screen angles), the tangent of the screen angle is approximated to a rational number.
The following is a summary description of the technology disclosed in Japanese Examined Patent Publication Number 52-49361, with reference to FIG. 1. In the figure, a supertile is made up of a group of half-tone cells formed roughly in the shape of a square and comprising 17 image elements each. The screen angle is the arctangent of 1/4 (14.04 degrees, which is roughly 15 degrees).
In the drawing, the image elements numbered (1, 2, . . . , 17) are assigned ascending threshold values. Thus, as the image data levels become higher, a dot pattern is obtained that starts at the center of the halftone cells and appears to grow toward the edges.
In the illustrated pattern, it can be seen that the same pattern is repeated in both the primary scan direction and the secondary scan direction for each set of L image elements (L=17). Thus, a threshold memory of 17.times.17 words can be set up, and the contents of this memory can be repeatedly read so that an output image of an arbitrary size can be converted into halftone image data using this screen pattern.
According to the technology disclosed in Japanese Laid-open Publication Number 54-18302, the pattern shown in FIG. 1 can be further divided up into repeating patterns. The following is a description of this technology, with reference to FIG. 2. In FIG. 2, a rectangular area is examined, where this rectangle is made up of P image elements in the secondary scan direction (P=1 in this case) and L image elements in the primary scan direction (L=17 in this case).
The overall pattern in FIG. 2 is equivalent to repetitions of this rectangular block, where the block is repeated every L picture elements in the primary scan direction. In the secondary scan direction, the repeating blocks are shifted by S picture elements for each increment in the secondary scan direction. Thus, the same halftone image data can be obtained by reading from a threshold memory made up of 17.times.1 words while performing appropriate shifts. The image element counts L, P, and S are hereinafter referred to as the L parameter, the P parameter, and the S parameter, and these will be referred to collectively as the "cyclic parameters".
In the cases shown in FIG. 1 and FIG. 2, the screen angle can be approximated to an ideal value by increasing the size of the halftone cells. However, this decreases the number of lines that can be reproduced per unit length (hereinafter referred to as the line count). On the other hand, using a smaller halftone cell will increase the line count but will also introduce increased error in the actual screen angle relative to the ideal screen angle, which can result in moire patterns.
Japanese Laid-open Publication Number 3-187676 (U.S. patent application Ser. No. 434,924) and Japanese Laid-open Publication Number 5-110835 (U.S. patent application Ser. No. 652,927) disclose a technology where a supertile is divided up into a plurality of halftone cells, and dot patterns are generated for each halftone cell.
According to these technologies, increasing the size of the supertile results in a screen angle that can be closer to an ideal value, while relative high line counts can be maintained since the dot patterns are generated for small halftone cells.
FIG. 15 shows an example of such a supertile. In this example, 1 supertile is made up of 9 halftone cells. By repeating this supertile, a prescribed image area can be covered without gaps, as shown in FIG. 4. The sizes of the halftone cells do not need to be all identical. As an example, FIG. 3 shows a supertile made up half-tone cells which have image element counts of 231 as well as 232.
FIG. 7(a) shows the image region in FIG. 4 divided up by supertiles. The pattern number of the threshold value matrix pattern in each supertile is also shown. In conventional image processing devices, there is only 1 type of threshold value matrix pattern having a prescribed shape and image element count. Thus, the same pattern number (P1) is used throughout.
FIG. 7(b) shows an example of a dot pattern generated using the screen pattern from FIG. 7(a). In FIG. 7(b), there is a wider gap in the dot pattern occurring at a period three times that of the halftone cell. Visually, the image appears to have stripes. The reason for this is that the difference in areas between halftone cells is greater (in the example shown in FIG. 4, 1/14, which is approximately 7.1%), and also that the differences in the distance between the "centers of gravity" of the halftone cells are greater.
Thus, when the difference between the output resolution and the screen line count is small, the technology that involves simply dividing a supertile into halftone cells will generate low-frequency periodic structures. Furthermore, in the technology described above, the halftone cell is made relatively small, and this decreases the number of tone levels. These problems could be eliminated by increasing the size of the halftone cell, but this would unavoidably decrease the line count.